C1235

This PDF is a selection from an out-of-print volume from the National
Bureau of Economic Research

Volume Title: Determinants of Investment Behavior

Volume Author/Editor: Robert Ferber, ed.

Volume Publisher: UMI

Volume ISBN: 0-87014-309-3

Volume URL: http://www.nber.org/books/ferb67-1

Publication Date: 1967

Chapter Title: The Theory of Investment Behavior

Chapter Author: Dale Jorgenson

Chapter URL: http://www.nber.org/chapters/c1235

Chapter pages in book: (p. 129 - 188)

The Theory of Investment Behavior

DALE W. JORGENSON
UNIVERSITY OF CALIFORNIA AT BERKELEY

1. introduction
Business investment behavior is one of the areas of modern economic
research that is being studied most intensively; empirical studies are
accumulating rapidly,' and at the same time important developments

1
A very detailed review of the literature through 1960 has been provided by
R. Eisner and R. Strotz, "The Determinants of Business Investment," in D. B.
Suits, et al., impacts of Monetary Policy, Englewood Cliffs, 1963, pp. 60—338. A
more concise review of developments through 1962 has been presented by E. Kuh,
"Theory and Institutions in the Study of Investment Behavior," American Eco-
nomic Review, May 1963, pp. 260—268. Empirical studies published since 1962
include: S. Almon, "The Distributed Lag between Capital Appropriations and
Expenditures," Econometrica, January 1965, pp. 178—196; W. H. L. Anderson,
Corporate Finance and Fixed investment, Boston, 1964; A. Bourneuf, "Invest-
ment, Excess Capacity, and Growth," American Economic Review, September
1964, pp. 607—625; R. Eisner, "Investment: Fact and Fancy," American Economic
Review, May 1963, pp. 237—246; Eisner, "Capital Expenditures, Profits, and the
Acceleration Principle," Models of Income Determination, Studies in Income and
Wealth 28, Princeton University Press for National Bureau of Economic Research,
1964, pp. 137—176; Eisner, "Realization of Investment Anticipations," in J. S.
Duesenberry, E. Kuh, G. Fromm, and L. R. Klein, eds., The Brookings Quarterly
Econometric Model of the United States, Chicago, 1965; E. Greenberg, "A Stock-
Adjustment Investment Model," Econometrica, July 1964, pp. 339—357; B. Hick-
man, Investment Demand and U.S. Economic Growth, Washington, 1965; D. W.
Jorgenson, "Capital Theory and Investment Behavior," American Economic Review,
May 1963, pp. 247—259; Jorgenson, "Anticipations and Investment Behavior," in
Brookings Quarterly Econometric Model; E. Kuh, Capital Stock Growth: A Micro-
Econometric Approach, Amsterdam, 1963; J. R. Meyer and R. R. Glauber, invest-
ment Decisions, Economic Forecasting and Public Policy, Boston, 1964; G. J. Stigler,
Capital and Rates of Return in Manufacturing Industries, Princeton for NBER, 1963.

130 Anatomy of Investment Behavior
in the economic theory of investment behavior are taking place.2 As yet,
there is very little common ground between the empirical and theoretical
approaches to this subject. From a certain point of view this is a
desirable state of affairs.3 Econometric studies of investment behavior
date back no more than thirty years.4 Only recently have data on invest-
ment expenditures suitable for analysis by econometric methods become
available. If empirical studies are forced prematurely into a theoretical
straitjacket, attention may be diverted from historical and institutional
considerations that are essential to a complete understanding of invest-
ment behavior. On the other hand, if theoretical work is made to con-
form to "realistic" assumptions at too early a stage in the development
of empirical work, the door may be closed to theoretical innovations
that could lead to improvements in empirical work at a later stage.
While there is some surface plausibility in the view that empirical
and theoretical research are best carried out in isolation from each
other, this view is seriously incomplete. Econometric work is always
based on highly simplified models. The number of possible explanations
of investment behavior, which is limited only by the imagination of the
investigator, is so large that, in any empirical investigation, all but a
very few must be ruled out in advance. Insofar as the necessary simplifi-
cations restrict the possible explanations of investment behavior, these
simplifications constitute, at least implicitly, a theory of investment
behavior. Such theories can be compared with each other most expedi-
tiously by reducing each to its basic underlying assumptions, after which
empirical tests to discriminate among alternative theories can be
2 See, for example, the following papers: K. J. Arrow, "Optimal Capital Policy,
The Cost of Capital, and Myopic Decision Rules," Annals of the Institute of Sta-
tistical Mathematics, 1964, pp. 2 1—30; "Optimal Capital Adjustment," in K. J.
Arrow, S. Karlin, and H. Scarf, eds., Studies in Applied Probability and Manage-
ment Science, Stanford, 1962; K. J. Arrow, M. Beckmann, and S. Karlin, "Opti-
mal Expansion of the Capacity of the Firm," in K. J. Arrow, S. Karlin, and H.
Scarf, eds., Studies in the Mathematical Theory of Inventory and Production,
Stanford, 1958;. A. S. Manne, "Capacity Expansion and Probabilistic Growth,"
Econometrica, October 1961, pp. 632—649; E. Zabel, "Efficient Accumulation of
Capital for the Firm," Econometrica, January-April 1963, pp. 13 1—150; and the
following books: T. Haavelmo, A Study in the Theory of Investment, Chicago,
1960; F. A. Lutz and D. G. Hague, eds., The Theory of Capital, London, 1961;
P. B. D. Optimal Investment Decisions, Englewood Cliffs, 1962; V. L.
Smith, Investment and Production, Cambridge, 1961; 13. Thalberg, "A Keynesian
Model Extended by Explicit Demand and Supply Functions for Investment
Goods," Stockholm Economic Studies, Pamphlet Series, No. 3, 1964.
3 This point of view has been put forward by K. Borch, "Discussion," American
Economic Review, May 1963, pp. 272—274.
J. Tinbergen, Statistical Testing of Business Cycle Theories, Part I, "A Method
and its Application to Investment Activity," Geneva, 1939.

Theory of InvestmenL Behavior 131

designed. Far from forcing empirical studies into a theoretical strait-
jacket, judicious use of a theoretical framework is essential to the proper
direction of empirical work.
The view that theoretical and empirical research should be carried
out in isolation is incomplete in a second respect. The use of economic
theory as a source of possible explanations for investment behavior frees
econometric work from reliance on empirical generalizations that have
not been subjected to rigorous econometric tests. There is a very real
danger that econometric models of investment behavior may be made
to conform prematurely to assumptions that are "realistic" by the
standards of empirical work not based on econometric methods. Just as
premature reliance on "realistic" assumptions may be stultifying to the
development of economic theory, so reliance on historical and institu-
tional generalizations may restrict the development of econometric
models unduly. The paramount test for "realism" of an econometric
model is its performance in econometric work. If a model does not per-
form satisfactorily by the standards of econometrics, it must be rejected,
however closely it parallels historical and institutional accounts of the
same economic behavior.
The point of departure for this paper is that progress in the study of
investment behavior can best be made by comparing econometric models
of such behavior within a theoretical framework. Ideally, each model
should be derived from a common set of assumptions about the objec-
tives of the business firm. Differences among alternative models should
be accounted for by alternative assumptions about the behavior of
business firms in pursuing these objectives. It will undoubtedly be sur-
prising to some that a theoretical framework is implicit in the econo-
metric models of investment behavior currently under study. The objec-
tive of this paper is to make this framework explicit in order to provide
a basis to evaluate evidence on the determinants of investment behavior.
This objective can only be attained by a thoroughgoing reconstruction of
the theory of investment. Once the theory of investment is placed in a
proper setting, the arguments advanced for pessimism about combining
theoretical and empirical work largely evaporate.
In providing a framework for the theory of investment behavior, the
first problem is to choose an appropriate basis for the theory. Two alter-
native possibilities may be suggested. First, the theory of investment
could be based on the neoclassical theory of optimal capital accumulation.
There are three basic objections to this possibility, the first of which is
that a substantial body of noneconometric work on the motivation of
business firms, mainly surveys of businessmen, suggests that "mar-

ginalist" considerations are largely irrelevant to the making of business
decisions. This evidence has been subjected to careful scrutiny by
White,5 who concludes that the data accumulated by the surveys are so
defective, even by the standards of noneconometric empirical work,
that no reliance can be placed on conclusions based on them. A second
objection is that previous attempts to base the study of investment on
neoclassical economic theory have been unsuccessful,° but this argument
will not withstand critical scrutiny. First, none of the tests of the neo-
classical theory reported in the early literature was based on a fully
rigorous statement of the theory. Secondly, the assumptions made about
the lag between changes in the demand for capital services and actual
investment expenditures were highly restrictive. Frequently, the lag
was assumed to be concentrated at a particular point or to be distributed
over time in a very simple manner. Tests of the neoclassical theory were
carried out prior to the important contribution of Koyck to the analysis
of distributed lags and investment behavior.7 Despite these deficiencies,
the pioneering tests of the neoclassical theory reported by Tinbergen
reveal substantial effects for the price of investment goods, the change
in this price, and the rate of interest.8 Similarly, tests reported by Roos
reveal substantial effects for the price of investment goods and rate of
interest.9 Klein's studies of investment in the railroad and electric power
industries reveal substantial effects for the rate of interest.10
A third and more fundamental objection has recently been restated
by Haavelmo, who argues that a demand schedule for investment goods
cannot be derived from neoclassical theory:11
What we should reject is the naive reasoning that there is a demand schedule
for investment which could be derived from a classical scheme of producers'

W. H. White, "Interest Inelasticity of Investment Demand," American Eco-
nomic Review, September 1956, pp. 565—587.
6 J• Meyer and E. Kuh, The In vestment Decision, Cambridge, Mass., pp. 7—14.

L. M. Koyck, Distributed Lags and Investment Analysis, Amsterdam, 1954.
8 Tinbergen, Statistical Testing, see also the discussion of Tinbergen's results by

T. Haavelmo, "The Effect of the Rate of Interest on Investment: A Note," Review
of Economic Statistics, February 1941, pp. 49—52.
C. F. Roos and V. S. Von Szeliski, "The Demand for Durable Goods,"
Econometrica, April 1943, pp. 97—122; Roos, "The Demand for Investment
Goods," American Economic Review, May 1948, pp. 311—320; Roos, "Survey of
Economic Forecasting Techniques," Econometrica, October 1955, pp. 363—395,
10 L. R. Klein, "Studies in Investment Behavior," in Conference on Business
Cycles, New York, National Bureau of Economic Research, 1951.
11 Haavelmo, Theory of Investment, p. 216.

Theory of Investment Behavior 133
behavior in maximizing profit. The demand for investment cannot simply be
derived from the demand for capital. Demand for a finite addition to the
stock of capital can lead to any rate of investment, from almost zero to
infinity, depending on the additional hypothesis we introduce regarding the
speed of reaction of capital-users. J think that the sooner this naive, and
unfounded, theory of the demand-for-investment schedule is abandoned, the
sooner we shall have a chance of making some real progress in constructing
more powerful theories to deal with the capricious short-run variations in the
rate of private investment.
We will show that it is possible to derive a demand function for invest-
ment goods based on purely neoclassical considerations. While it is true
that the conventional derivation of such a demand schedule, as in
Keynes' construction of the marginal efficiency of investment schedule,12
must be dismissed as naive, there is a sense in which the demand for
investment goods can be taken to depend on the cost of capital; such
a theory of investment behavior can be derived from the neoclassical
theory of optimal capital accumulation.
A second possible basis for the theory of investment is the assumption
that business firms maximize utility defined more broadly than in the
characterization of objectives of the firm in the neoclassical theory of
optimal capital accumulation. This basis has been suggested by Meyer
and Kuh:13
Partial recognition of institutional changes has led in recent years to shift the
theory of the firm, and consequently of plant and equipment investment,
from a profit maximization orientation to that of utility maximization. Pri-
marily, this move represents a growing belief that profit maximization is too
narrow to encompass the full scope of modern entrepreneurial motives, par-
ticularly once the previously assumed objective conditions are released from
ceteris paribus, and the theory seeks to explain a much wider range of
behavioral responses.

This position has recently been supported with much force by Simon:
•. I should like to emphasize strongly that neither the classical theory
of the firm nor any of the amendments to it or substitutes for it that
have been proposed have had any substantial amount of empirical test-
ing. If the classical theory appeals to us, it must be largely because it

12 M. Keynes, The General Theory of Employment, Interest and Money,
New York, 1936, esp. Chapter 11, pp. 135—146.
18 Meyer and Kuh, Investment Decision, p. 9.

has a certain face validity . . rather than because profit maximizing
.

behavior has been observed."4
In putting forward this view, Simon ignores the entire econometric
literature on cost and production functions, all of which is based on the
neoclassical theory of the firm. A recent survey of this literature by
Walters'5 enumerates 345 references, almost all presenting results of
econometric tests of the neoclassical theory of the firm which are over-
whelmingly favorable to the theory. The evidence is largely so favorable
that current empirical research emphasizes such technical questions as
the appropriate form for the production function and the appropriate
statistical specification for econometric models of production based on
this theory. We conclude that Simon's statement that the alternatives to
the neoclassical theory of the firm have had no substantial amount of
empirical testing is correct. However, his characterization of the empir-
ical evidence on the neoclassical theory is completely erroneous.
One possible reaction to a proper assessment of the support for the
neoclassical theory of the firm from econometric studies of cost and pro-
duction functions is to reject out of hand studies of investment behavior
not based explicitly on the neoclassical theory, such as the study of
Meyer and Kuh. In fact, the theoretical basis for the econometric model
of investment behavior proposed by Meyer and Kuh is consistent with
the neoclassical theory of optimal capital accumulation. Their appeal to
a less narrow view of entrepreneurial objectives is not essential to the
interpretation of the empirical results they present. We conclude that the
objections to the neoclassical theory of the firm as a basis for the theory
of investment behavior are ill-founded. Furthermore, the appeal to a
broader view of entrepreneurial objectives than that which underlies this
theory is not required by evidence either from econometric studies of
cost and production functions or from studies of investment behavior.
The neoclassical theory of optimal accumulation of capital is a far more
powerful theory than the "broader view" suggested by Simon and others
in the sense that a much narrower range of conceivable behavior is
consistent with it than with the amorphous utility-maximizing theory.
Accordingly, we will employ a theoretical framework based on the
neoclassical theory of the firm for constructing a theory of investment
behavior.

'4 H. A. Simon, "New Developments in the Theory of the Firm," American
Economic Review, May 1962, p. 8.
15 A. A. Walters, "Production and Cost Functions: An Econometric Survey,"
Econometrica, April 1963, pp. 1—66.

The objective of explaining investment behavior on the basis of the
neoclassical theory of the firm cannot be described as novel. This objec-
tive is clearly in evidence in Tinbergen's pioneering monograph, Statistical
Testing of Business Cycle Theories. Subsequently, a similar objective was
adopted by Roos and by Klein.16 In these early studies of investment
behavior, the neoclassical theory was employed to provide a list of pos-
sible explanatory variables for investment expenditures. The rate of
interest, the level of stock prices, the price of investment goods, and
changes in the price of investment goods were used along with other
variables such as profits, output, and changes in output. Little attention
was paid to the manner in which the rate of interest and the price of
investment goods enter the demand for capital services or the demand for
investment goods. Both variables enter only through the user cost of
capital services.'7 There is no effect of the price of investment goods
except in combination with the rate of interest and vice versa. We con-
clude that, although the objective of explaining investment behavior on
the basis of the neoclassical theory of the firm is not new, this objective
remains to be fully realized.

2. The Neoclassical Framework
In formulating a theory of investment behavior based on the neoclassical
theory of optimal capital accumulation, a great number of alternative
versions of the theory could be considered. Reduced to its barest essen-
tials, the theory requires only that capital accumulation be based on the
objective of maximizing the utility of a stream of consumption. This
basic assumption may be combined with any number of technological
possibilities for production and economic possibilities for transforma-
tion of the results of production into a stream of consumption. In select-
ing among alternative formulations, a subsidiary objective must be borne
in mind. The resulting theory of capital accumulation must include the
principal econometric models of investment behavior as specializations,
18 See footnotes 9 and 10. See also L. R. Klein, The Keynesian Revolution,
New York, 1947, esp. pp. 62—68, pp. 196—199; Klein, "Notes on the Theory of
Investment," Kykios, vol. 2, Fasc. 2, 1948, PP. 97—117; Klein, Economic Fluctua-
tions in the United States, 1921—1 941, New York, 1950, esp. pp. 14—40.
17 A complete discussion of the concept of user cost has been given by W. A.

Lewis, "Depreciation and Obsolescence as Factors in Costing," in J. L. Meij, ed.,
Depreciation and Replacement Policy, Amsterdam, 1961, pp. 15—45. See also
Keynes, General Theory, pp. 66—73; A. P. Lerner, "User Cost and Prime User
Cost," American Economic Review, March 1943, pp. 131—132; F. A. Lutz and
V. Lutz, The Theory of Investment of the Firm, Princeton, 1951; A. D. Scott,
"Notes on User Cost," Economic Journal, June 1953, pp. 364—384.

but the theory need not encompass possibilities for the explanation of
investment behavior not employed in econometric work.
The essentials of a theory of optimal capital accumulation that meets
this basic objective are the following: The firm maximizes the utility of a
consumption stream subject to a production function relating the flow of
output to flows of labor and capital services. The firm supplies capital ser-
vices to itself through the acquisition of investment goods; the rate of
change in the flow of capital services is proportional to the rate of acquisi-
tion of investment goods less the rate of replacement of previously acquired
investment goods. The results of the productive process are transformed
into a stream of consumption under a fixed set of prices for output, labor
services, investment goods, and consumption goods. These prices may
be considered as current or "spot" prices together with forward prices
for each commodity or, alternatively, as current and future prices
together with a normalization factor, which may be identified with cur-
rent and future values of the rate of time discount or interest rate. Both
current and forward prices are taken as fixed by the firm. Alternatively,
current and future prices together with current and future values of the
rate of interest are taken as fixed. Under these conditions, the problem
of maximizing utility may be solved in two stages. First, a production
plan may be chosen so as to maximize the present value of the produc-
tive enterprise. Secondly, consumption is allocated over time so as to
maximize utility subject to the present value of the firm. In view of our
concern with the theory of business investment behavior, we will con-
sider only the first of these problems. It should be noted that, under the
assumption of fixed prices, the choice of a production plan is inde-
pendent of the subsequent allocation of consumption over time. Two
firms with different preferences among alternative consumption streams
will choose the same plan for production.
This version of the neoclassical theory of the firm is not the only one
available in the literature on capital theory. From a certain point of
view, the objective of maximizing the present value of the firm is only
one among many possible objectives for the firm. In a recent survey
paper on the theory of capital, Lutz remarks that "It is one of the sur-
prising things about capital theory that no agreement seems to have
been reached as to what the entrepreneur should maximize."8 Alterna-
tive criteria discussed in the literature include maximization of the
average internal rate of return, maximization of the rate of return on
capital owned by the firm, investment in any project with an internal
18 F. A. Lutz, "The Essentials of Capital Theory," in Lutz and Hague, Theory
of Capital, p. 6.

rate of return greater than the ruling market rate of interest, and so on.
None of these criteria can be derived from maximization of the utility
of a stream of consumption under the conditions we have outlined.
Maximization of the present value of the firm is the only criterion con-
sistent with utility maximization. This approach to the theory of optimal
capital accumulation was originated by Fisher and has recently been
revived and extended by Bailey and by Hirshleifer.'9 The essential justi-
fication for this approach is summarized by Hirshleifer, as follows:
Since Fisher, economists working in the theory of investment decision have
tended to adopt a mechanical approach—some plumping for the use of this
formula, some for that. From a Fisherian point of view, we can see that
none of the formulas so far propounded is universally valid. Furthermore,
even where the present-value rule, for example, is correct, few realize that
its validity is conditional upon making certain associated financing decisions
as the Fisherian analysis demonstrates. In short, the Fisherian approach per-
mits us to define the range of applicability and the short-comings of all the
proposed formulas—thus standing over against them as the general theoretical
solution to the problem of investment decision under conditions of certainty.20
A second controversial aspect of the version of the neoclassical theory
outlined above is the assumption that the set of technological possibilities
confronted by the firm can be described by a production function, where
the flow of output is a function of flows of labor and capital services
and the flow of capital services is proportional to the stock of capital
goods obtained by summing the stream of past net investments.21 The
concept of capital service is not essential to the neoclassical theory. A
production function relating output at each point of time to inputs of
labor and capital services at that point of time may be replaced by a
production function relating output at every point of time to inputs of
investment goods at every point of time; this description of the set of
19 Fisher, The Theory of Interest, New York, 1930. M. J. Bailey, "Formal
Criteria for Investment Decisions," Journal of Political Economy, October 1959,
pp. 476—488. J. Hirshleifer, "On the Theory of the Optimal Investment Decision,"
in E. Solomon, ed., The Management of Corporate Capital, Glencoe, 1959, pp.
205—228.
20 Ibid., p. 228.
21 For a discussion
of this assumption and some of its implications, see
J. Robinson, "The Production Function and the Theory of Capital," Review of
Economic Studies, Vol. 21, No. 54, 1953—54, pp. 81—106; R. M. Solow, "The
Production Function and the Theory of Capital," Review of Economic Studies,
Vol. 23, No. 61, 1955—56, pp. 101—108; J. Robinson, "Reply," Review of Eco-
nomic Studies, Vol. 23, No. 62, 1955—56, p. 247; J. Robinson, "Some Problems of
Definition and Measurement of Capital," Oxford Economic Papers, June 1959,
pp. 157—166; K. J. Arrow et a!., "Symposium on Production Functions and Eco-
nomic Growth," Review of Economic Studies, June 1962.

q

production possibilities is employed by Fisher; moreover, it may be char-
acterized abstractly so that even the notion of a production function may
be dispensed with, as is done by Malinvaud.22 The description of the set
of technological possibilities by means of a production function as pre-
sented by Fisher is a specialization of the description given by Malinvaud.
The further assumption that the relationship between inputs of invest-
ment goods and levels of output may be reduced to a relationship between
output at each point of time and a corresponding flow of capital services
involves a specialization of the description of technological possibilities
given by Fisher.
In the neoclassical literature, two basic models of the relationship
between flows of investment goods and flows of capital services have
been discussed, namely, a model of inventories and a model of durable
goods. At the level of abstraction of Fisher's description of the set of
production possibilities, no distinction between inventories and durable
goods is required. For both inventories and durable goods, the acquisi-
tion of a stock of productive goods may be represented as an input to
the productive process at the time of acquisition. For inventories, the
individual items "used up" at different points of time may be represented
as the output of a subprocess representing the holding of stocks; these
outputs may be inputs into other subprocesses. For durable goods, the
outputs of the corresponding stockholding process are the services of the
goods rather than the individual items of the stock; the services of the
durable goods may be inputs into other parts of the productive process.
The basis for the distinction between inventories and durable goods
lies in the relationship among the initial input and the various outputs
from the stockholding process. For inventories, the outputs provided by
the stockholding process are customarily treated as perfect substitutes.
For each item held in stock, the ultimate consumption of that item can
occur at one and only one point in time. By contrast, the outputs pro-
vided by durable goods are treated as if they were perfectly comple-
mentary. The output of the service of a durable good at any point of
time is assumed to bear a fixed relation to the output of the same service
at any other point of time. The assumptions that outputs provided by a
given input of investment goods are perfectly complementary or per-
fectly substitutable are highly restrictive. Nevertheless, the simplification
of the neoclassical theory for these limiting cases and the practical
importance of these cases are very great. A far more substantial propor-
tion of the literature on capital theory is devoted to these two limiting
22 E. Malinvaud, "Capital Accumulation and Efficient Allocation of Resources,"
Econometrica, April 1953, pp. 233—268.

cases than to the theory of production at the level of abstraction of the
descriptions of technology given by Fisher or by Malinvaud. In the
following we assume that the conventional neoclassical description of a
durable good is appropriate for each investment good considered.
A second assumption required for a relationship between output at
each point of time and the corresponding flow of capital services is that
the services of investment goods acquired at different points of time are
perfect substitutes in production. Accordingly, the flow of capital ser-
vices from each investment good is proportional to the stock of capital
that may be obtained by simply adding together all past acquisitions less
replacements. This assumption is highly restrictive; the assumption can
be justified primarily by the resulting simplification of the neoclassical
theory. We discuss only a single investment good. Under the assump-
tions outlined above, there is only a single capital service. This simplifi-
cation is also completely inessential to neoclassical theory.
Finally, we assume that the flow of replacement generated by a given
flow of investment goods is distributed over time in accord with an
exponential distribution. This assumption implies that the flow of
replacement investment at any point of time is proportional to the
accumulated stock of investment goods. Again, this assumption is only
one among many possibilities. Alternative assumptions employed in
practice include the following: First, replacement is equal to investment
goods acquired at some earlier point in time; second, replacement is
equal to a weighted average of past investment flows, with weights
derived from studies of the "survival curves" of individual pieces of
equipment.23 For empirical work the exponential distribution of replace-
ments is of special interest. While empirical studies of "survival curves"
for individual pieces of equipment reveal a wide variety of possible
distributions, there is a deeper justification for use of the exponential
distribution. This justification arises from a fundamental result of
renewal theory, namely, that replacement approaches an amount pro-
portional to the accumulated stock of capital whatever the distribution
of replacements for an individual piece of equipment, provided that the
size of the capital stock is constant or that the stock is growing at a
constant rate (in the probabilistic sense) •24 This asymptotic result may
be used as the basis for an approximation to the distribution of replace-
23 A summary of research on the lifetimes of capital equipment as given by
A. Marston, R. Winfrey, and J. C. Hempstead, Engineering Evaluation and
Depreciation, 2nd ed., New York, 1953.
24 For a statement of the basic theorem, see E. Parzen, Stochastic Processes,
San Francisco, 1962, pp. 180—181.

ments; for any investment good, the stream of replacements eventually
approaches a stream that would be generated by an exponential distri-
bution of replacements. Accordingly, the exponential distribution may
be used as an approximation to the distribution of replacements for the
purpose of estimating the stream of replacements. A simple indirect test
of the validity of this approximation has been carried out by Meyer and
Kuh.25 For any distribution of replacements except the exponential dis-
tribution, one would expect to observe an "echo effect" or bunching of
replacements at lags corresponding to points of relatively high density
in the conditional distributions of replacements for individual types of
equipment. Meyer and Kuh report no evidence for such an effect.
To summarize, we consider a version of the neoclassical theory in
which the objective of the firm is maximization of its present value. This
may be derived from the objective of maximizing the utility of a con-
sumption stream subject to a fixed set of production possibilities and to
fixed current and future prices and interest rates. Since the choice of a
production plan is entirely independent of the corresponding choice of a
consumption stream, two individuals with different preferences among
consumption streams will choose the same production plan. Secondly,
we consider a description of technological possibilities in which output
at each point of time depends on the flow of labor and capital services
at that point of time, the flow of capital services is proportional to the
stock of capital goods, and replacements are also proportional to the
stock of capital goods. This description of technology is a specialization
of the descriptions given by Malinvaud and by Fisher. The essential
justffication for this specialization is that the resulting theory of optimal
capital accumulation is sufficiently broad to include the principal econo-
metric models of investment behavior as special cases.

3. Optimal Capital Accumulation
To develop the theory of investment behavior in more detail, we must
first define the present value of the firm. For simplicity, we limit the
analysis to a production process with a single output, a single variable
input, and a single capital input. Where Q, L, and I represent levels of
output, variable input, and investment in durable goods and p, w, and q
represent the corresponding prices, the flow of net receipts at time t, say
R(t), is given by:
R(t) = p(t)Q(t) — w(t)L(t) — q(t)!(t). (1)
25 Meyer and Kuh, Investment Decision, pp. 91—94.

Present value is defined as the integral of discounted net receipts; where
r(s) is the rate of time discount at time s, net worth (W) is given by
the expression:

W d8 P(t) di. (2)
= f e—Jr(8)
For purposes of the following discussion, we may assume that the time
rate of discount is a constant without loss of generality. Accordingly,
the present value of the firm may be represented in the simpler form:

w= f dt.

Present value is maximized subject to two constraints. First, the rate
of change of the flow of capital services is proportional to the flow of
net investment. The constant of proportionality may be interpreted as
the time rate of utilization of capital stock, that is, the number of units
of capital service per unit of capital stock. We will assume that capital
stock is fully utilized so that this constant may be taken to be unity. Net
investment is equal to total investment less replacement; where replace-
ment is proportional to capital stock, this constraint takes the form:
K(t) = 1(t) — oK(t) (3)
where K(t) is the time rate of change of the flow of capital services at
time t. This constraint holds at each point of time so that K, K, and I
are functions of time; to simplify notation, we will use K in place of
K(t), 1 in place of 1(t), and so on. Secondly, levels of output and levels
of labor and capital services are constrained by a production function:
F(Q,L,K) = 0. (4)
We assume that the production function is twice differentiable with
positive marginal rates of substitution between inputs and positive
marginal productivities of both inputs. Furthermore, we assume that
the production function is strictly convex.
To maximize present value (2) subject to the constraints (3) and
(4), we consider the Lagrangian expression:

.2? = f [e r tR(t) + x 0(t)F( Q, L, K) + x 1(t)(k — I + ÔK)] di, (5)

= f f(t) dt,

•1

where:
f(t) = + X0(t)F(Q, L, K) + X1(t)(k — I + AK).
The Euler necessary conditions for a maximum of present value subject
to the constraints (3) and (4) are:

X0(t) — = 0, (6)

0.1
— + =

Of
— X1(t) = 0,

oj — dOf =
——
OF d
Xo(t)—- + &X1(t) — —X1(t) = 0,
OK diaK OK dt
and also:

=F(Q,L,K)=0, (7)
c9X0

=K—I+ oK=O.
cTh1

Combining the necessary conditions for labor and output, we obtain
the marginal productivity condition for labor services:
w
(8)
p
Of course, output, labor, wages, and prices are all functions of time.
The difference between this marginal productivity condition and the
corresponding condition of the "static" theory of the firm is that condi-
tion (8) holds at every point of time over the indefinite future whereas
the marginal productivity condition of the "static" theory of the firm
holds only at a single point in time. A similar marginal productivity
condition for capital services may be derived. First, solving the neces-
sary conditions (6) for Xi(t):
X1(t) =
the necessary condition for capital services may be written:
OF
Xo(t) —i — — + = 0.

a.

Combining this condition with the necessary condition for output, we
obtain the marginal productivity condition for capital services:

3Q q(r+b)—qc
p p' ()
where:
(10)

Again, output, capital, prices, and the rate of time discount are functions
of time so that these conditions hold at every point of time over the
indefinite future.
Expression (10) defines the implicit rental value of capital services
supplied by the firm to itself. This interpretation of the price c(t) may
be justified by considering the relationship between the price of capital
goods and the price of capital services. First, the flow of capital services
over an interval of length dt beginning at time t from a unit of invest-
ment goods acquired at time s is:

e&(t8) dt.

If c(t) is the price of capital services at time t, then the discounted price
of capital services is so that the value of the stream of capital
services on the interval dt is:

dt.

Similarly, if q(s) is the price of capital goods at time s, then the discounted
price of capital goods is so that the value of a unit of investment
goods acquired at time s is:

But the value of investment goods acquired at time s is equal to the
integral of the discounted value of all future capital services derived from
these investment goods:

eraq(s) dt,
= J 8

= e68J e_(r+o)tc(t) dt.
8

Solving for the price of capital goods, we obtain:

q(s) = e(r)8 dt,

= f dt.

To obtain the price of capital services implicit in this expression, we
differentiate with respect to time:
= [r(s) + — c(s),
so that:
c = q(r + —

which is expression (10) given above for the implicit rental value of capital
services.
The conditions describing the neoclassical model of optimal capital
accumulation may also be derived by maximization of the integral of
discounted profits, where profit at each point of time, say, P(t), is given by:
P(t) = p(t)Q(t) — w(t)L(t) — c(t)K(t). (11)
The integral of discounted profits, say, is given by the expression:

W÷ e_rtP(t) dt. (12)
= J 0

The side condition for investment may be disregarded, since investment
does not enter into the definition of profit (11); substituting the side
condition for the shadow price of capital services into the profit function,
we obtain:

= f — w(t)L(t) — (q(t)[r(t) + ô] — di.

To maximize this function subject to the production function, it suffices
to maximize profit at each point of time subject to the production func-
tion. But this yields the marginal productivity conditions (8) and (9)
and the production function (4) itself. Reintroducing the side condi-
tions (3) and (10), we obtain the complete neoclassical model of
optimal capital accumulation.
The integral of discounted profits is not the same as the integral
defining present value of the firm. The difference between the two is
given by:

-t I - —

Theory of investment Behavior 145

W —
= j 0
— P(t)J dt

= f e_rt[{q(t)[r(t) + o] — — q(t)I(i)] dt

= f — dt
+ q(t)r(t)K(t) — cj(t)K(t) — q(t)K(t)

= q(O)K(O),

which is the value of capital stock on hand at the initial point of time.
The present value of the firm is the sum of the integral of discounted
profits and the market value of the assets of the firm. Since the market
value of the assets of the firm is fixed, maximization of the integral of
discounted profits results in the same path for accumulation of capital
as maximization of present value of the firm. To summarize, the neo-
classical model of optimal capital accumulation may be derived by
maximizing present value of the firm, by maximizing the integral of
discounted profits of the firm, or simply by maximizing profit at each
point of time.
In taking maximization of profit as the objective of the firm, profit
is defined in a special sense, namely, net receipts on current account less
the implicit rental value of capital services. This concept of profit would
agree with the usual accounting definition of profit only in rather unusual
circumstances, for example, where the firm actually rents all the capital
services it employs. The price of capital services is then a market price
and the rental value of the services is an actual outlay. Where the firm
supplies capital services to itself, the implicit rental value of capital
services c(t) is a shadow price which may be used by the firm in the
computation of an optimal path for capital accumulation. For optimal
capital accumulation, the firm should charge itself a price for capital
services equal to the implicit rental value and should then maximize
profit at each point of time in the usual way. It is very important to note
that the conditions determining the values of each of the variables to be
chosen by the firm—output, labor input, and investment in capital
goods—depend only on prices, the rate of interest, and the rate of
change of the price of capital goods for the current period. Accordingly,
in the neoclassical theory of optimal capital accumulation, the firm
behaves at each point of time as in the "static" theory of the firm,
provided that the price of capital services is taken to be equal to the

146 Anatomy 0 Investment Behavior
corresponding implicit rental value. Of course, in the "static" theory the
marginal productivity condition (9) holds only at a single point in time.
The complete neoclassical model of optimal capital accumulation
consists of the production function (4) and the two marginal produc-
tivity conditions (8) and (9):
w 9Q c
F(Q,K,L)=O, —=—,
p
9L OK p
and the two side conditions (3) and (10):
I=K+öK,
c =q(r+
The production function and marginal productivity conditions hold at
each point of time. The side conditions are differential equations also hold-
ing at each point of time. Combined, these conditions determine the levels
of output, labor input, and capital input, together with the level of
investment and the shadow price for capital services.
The interpretation of condition (3) determining the level of invest-
ment is the source of some diffIculty in the literature. If the level of
investment is bounded, the derivative of the level of capital services
must be bounded. But this implies that the level of capital services itself
must be continuous. Since we have assumed that the production func-
tion is twice differentiable, a sufficient condition for continuity of the
level of capital services is continuity of the prices—w, p. c.
One interpretation of condition (3) is that the initial value of the
level of capital services may be chosen arbitrarily. This interpretation
has been suggested by Haavelmo and by Arrciw.26 If the initial level of
capital services is derived from the production function and the marginal
productivity conditions and if the initial value of capital is fixed arbi-
trarily, optimal capital accumulation may require an unbounded initial
level of investment. In management science, this interpretation of the
problem may be of some interest, though even there the interpretation
seems somewhat forced, as Arrow points out.27 For empirical work this
interpretation is completely artificial since firms are viewed as making
new decisions to invest continuously over time. To maximize present
value at each point of time, a firm following an optimal path for capital
accumulation must maximize present value subject to the initial condi-
26 Haavelmo, Theory of Investment, pp. 162—165. Arrow, "Optimal Capital
Adjustment," in Studies in Applied Probability, p. 2.
271b1d., p.6, in. 1.

tion given by the optimal path up to that point. But this results in a new
optimal path which is precisely the same as the old from that point for-
ward. Accordingly, if the optimal path for capital accumulation is con-
tinuous, the initial value of the level of capital services may not be
chosen arbitrarily in the maximization of the present value of the firm.
At each point it is precisely that for which the initial level of investment
is bounded, namely, the level of capital services derived from the pro-
duction function and the marginal productivity conditions. A possible
objection to this view is that firms must begin to accumulate capital at
some point in time. But at such a point the initial level of capital ser-
vices is not given arbitrarily; the initial level must be zero with a positive
derivative.

4. The Theory of investment Behavior
Beginning with the neoclassical model of optimal capital accumulation,
we may derive differentiable demand functions for labor and capital
services and a differentiable supply function for output, say:
L = L(w, c,p), (13)
K = K(w, c, p),
Q= Q(w, c, p).
The problem of deriving the demand for investment goods as a function
of the rate of interest is a subtle one. Haavelmo expresses the view that
the demand for investment goods cannot be derived from the profit-
maximizing theory of the firm. This is a consequence of his interpreta-
tion of the demand function for capital services and condition (3)
determining the level of investment from replacement and the rate of
change of demand for capital services. According to this interpretation,
finite variations in the rate of interest with all other prices held constant
result in finite changes in the demand for capital services. As the rate of
interest varies, demand for investment goods assumes only three possible
values—negatively infinite, positively infinite, or the value obtained
where the initial level of capital services is precisely equal to the demand
for capital services. Investment demand has a finite value for only one
rate of interest. In this interpretation, the demand function for capital
services is analyzed by means of comparative statics, that is, by com-
paring alternative production plans at a given point of time. Any attempt
to derive the demand for investment goods as a function of the rate of

interest by such comparisons leads to nonsensical results, as Haavelmo
correctly points out.
However, an alternative interpretation of the demand function for
capital services and condition (3) determining the level of investment is
possible. Under the hypothesis that the firm is following an optimal
path for capital accumulation and that the optimal path is continuous,
the initial level of capital is always equal to the demand for capital
services. By imposing this condition at the outset, the demand for
investment goods as a function of the rate of interest at any point of
time may be analyzed by means of comparative dynamics, that is, by
comparing alternative paths of capital accumulation, each identical up
to that point of time and each continuous at that point. The demand
for investment goods is given by condition (3):
i=k+ öK,
where the level of capital services, K, is fixed; but from the demand
function for capital services (13), this condition implies that for fixed
values of the price of output and the price of labor services, the implicit
price of capital services must remain unchanged. Holding the price of
investment goods constant, the rate of change of the price of investment
goods must vary as the rate of interest varies so as to leave the implicit
price of capital services unchanged. Formally, the condition that varia-
tions in the rate of interest leave the implicit price of capital services
unchanged may be represented as:
t9c
—=0;
holding the price of investment goods constant, this condition implies
that the own-rate of interest on investment goods, r — 4'/q, must be left
unchanged by variations in the rate of interest.
We assume that all changes in the rate of interest are precisely com-
pensated by changes in the rate of change of the price of current and
future investment goods so as to leave the own-rate of interest on invest-
ment goods unchanged. Under this condition the discounted value of all
future capital services, which is equal to the current price of investment
goods, is left unchanged by variations in the time path of the rate of
interest. The condition that the time path of the own-rate of interest on
investment goods is left unchanged by a change in the time path of the
rate of interest implies that forward prices or discounted future prices
of both investment goods and capital services are left unchanged by

variations in the rate of interest. For a constant rate of interest, this
condition may be represented in the form:
.92e_rtc(t)
=
är.9t

Like the previous condition, this condition holds at every point of time.
To derive the demand for investment goods as a function of the rate
of interest, we first differentiate the demand for capital services with
respect to time, obtaining:
.9K .9w 3K 3c .9K ap
K=—•—+—•—+—•—
.9w .9t .9c 3t .9p at

.9w op
For simplicity, we consider only the case in which — = — = 0, that is,
the price of output and the price of labor services are not changed. In this
case, we obtain:
.9K t9c

.9c .9t

Differentiating the implicit price of capital services with respect to time,
we have:
'3c ôq Or 02q
—=—(ô+r)+q———---- (14)
at at at .912

To derive the demand for investment goods, we combine expression
(14) for the rate of change of capital services with condition (3) for
the rate of investment, obtaining:
aKraq .9r a2ql
1=—I —(â+r)+q——---—
8c L.9t at
1+ .5K,

which depends on the rate of interest and the price of investment goods
through the rate of change of capital services. Differentiating this invest-
ment demand function with respect to the rate of interest, we obtain:
.91 .92K äc .9c .9K c92c .9K äc
= + a—.—
or .9c2 Or Ot Oc OtOr Oc Or

t3c
But — = 0, since changes in the rate of interest are compensated by
Or
changes in the rate of change of the price of investment goods so as to

leave the implicit price of capital services unchanged. This condition
implies that:
32q
=
atar
Secondly, = 0, since changes in the time path of the rate of
3rt9l
interest leave the time path of forward or discounted prices of capital
services unchanged. This condition implies that:
a 2c
= C.
Ot ar

Combining these two conditions, we obtain:
(91 (9K
— =—.c <0,
(9r 0c
so that the demand for investment goods is a decreasing function of the
rate of interest.
We conclude that it is possible to derive the demand for investment
goods as a function of the rate of interest on the basis of purely neo-
classical considerations. However, the demand for investment goods
depends on the rate of interest through a comparison of alternative paths
of capital accumulation, each continuous and each depending on a time
path of the rate of interest. Although this conclusion appears to be the
reverse of that reached by Haavelmo, his approach to the demand for
investment goods is through comparative statics, that is, through com-
parison of alternative production plans at a given point of time. The
demand function for investment goods cannot be derived by means of
such comparisons. As a proposition in comparative statics, any relation
between variations in the rate of investment and changes in the rate of
interest is nonsensical.
To summarize, the complete neoclassical model of optimal capital
accumulation consists of the production function (4), the two marginal
productivity conditions (8) and (9), and the side condition (10). An
alternative form of this model consists of the demand functions for
capital and labor services, the supply function for output:
L = L(w, c, p),
K = K(w, c, p),
Q = Q(w, c,p);
and the demand function for investment goods:

Theory of investment Behavior 151
ÔK ôc
ôc ät
/ i3c

The demand for investment goods depends on the change in the
demand for capital with respect to a change in the implicit price of
capital services, the time rate of change in the price of capital services,
and the level of replacement demand. Where the time rates of change
of the price of labor services and the price of output are not zero, the
demand function for investment goods may be rewritten:
aKaw aKac aK ap
I=——+——+——+oK,
aw at ôc ôt ap at
= f w, C, awacap
p, — ,—,—
at at at

5. Alternative Theories of investment Behavior
The neoclassical theory of demand for investment goods just outlined
may be contrasted with the theory current in the literature. Most recent
accounts of the theory of demand for investment are based on Keynes'
General Theory, in which the criterion for optimal investment behavior
is that any project with an internal rate of return greater than the ruling
rate of interest is undertaken.28 An investment demand schedule is con-
structed by varying the rate of interest and plotting the quantities of
investment undertaken for each value of the rate of interest. The cri-
terion for optimal investment behavior used by Keynes is inconsistent
with maximization of the present value of the firm, as Alchian and
Hirshleifer have pointed out.2° Nevertheless, a substantial portion of the
current literature on the investment demand function is based on a
straightforward reproduction of Keynes' derivation. Aichian lists a num-
ber of examples from the literature prior to 1955; examples from the

28 Chapter 11, "The Marginal Efficiency of Capital," especially p. 136.
29 A. A. Aichian, "The Rate of Interest, Fisher's Rate of Return over Costs and
Keynes' Internal Rate of Return," in Management of Corporate Capital, p. 70;
and J. Hirshleifer, in ibid., pp. 222—227. This conclusion of Aichian and Hirsh-
leifer contradicts the position taken by Klein in The Keynesian Revolution.

current literature are provided by the recent work of Duesenberry and
Tarshis.3° Keynes' construction of the demand function for investment
must be dismissed as inconsistent with the neoclassical theory of optimal
capital accumulation.
An alternative construction of the demand function for investment
goods has been suggested by Fisher.3' In Fisher's theory any project
with positive present value is undertaken. Keynes appears to have identi-
fied his construction of the marginal efficiency of capital schedule with
that of Fisher, as Aichian points out.32 There are two difficulties with
Fisher's construction. First, the construction is carried out by means of
comparative statics so that the resulting schedule may be interpreted as
a theory of demand for capital services for which no demand function
for investment goods exists. Second, the construction is not internally
consistent in a second sense pointed out by Aichian, since ". . we .

cannot in full logical consistency draw up a demand curve for invest-
ment by varying only the rate of interest (holding all other prices in the
impound of ceteris paribus) The relevant prices are forward prices
of all commodities; but altering the rate of interest amounts to altering
certain forward prices. It is inconsistent to vary the rate of interest
while holding such prices fixed. This inconsistency may be eliminated
by stipulating that variations in the rate of interest must be precisely
compensated by changes in the time rate of change of the price of
investment goods. The price of investment goods at a given point of
time is held fixed; the rate of change of the price of investment goods
varies with the rate of interest. The construction of the demand function
for investment goods involves a comparison among alternative paths of
optimal capital accumulation; all paths are identical up to the point of
time for which the investment function is constructed. Such a theory
of investment behavior is internally consistent and may be derived by
means of comparative dynamics.
Klein has attempted to derive a demand function for investment goods
on the basis of profit maximization. His treatment, though suggestive, is
30 J• S. Duesenberry, Business Cycles and Economic Growth, New York, 1958,
pp. 49—85. Duesenberry asserts that Keynes' derivation is based on "profit maxi-
mization" (p. 85). L. Tarshis, "The Marginal Efficiency Function," American
Economic Review, December 1961, pp. 958—985. Tarshis asserts that the Keynesian
theory is based on that of the "profit-maximizing firm" (pp. 958—959).
31 Fisher, Theory of Interest, pp. 159—176.
32 Aichian, in Management of Corporate Capital, p. 67. Klein (Keynesian
Revolution, p. 62) follows Keynes in identifying these two distinct approaches to
the construction of the marginal efficiency schedule.
Aichian, Management of Corporate Capital, p. 71.

marred by a number of inconsistencies. In his first attempt, the stock of
investment goods is defined as the integral of past flows of investment,
but the flow of investment is employed as a stock in the production
function and in the definition of "discounted profit."34 A second attempt
involves the identification of the flow of capital services with the flow
of depreciation.35 In both attempts, quantities measured as rates of capi-
tal service per unit of time are added to quantities measured as rates of
investment per unit of time, which is self-contradictory. This inconsistency
carries over to the empirical implementation of the resulting investment
function, where the price of investment goods is identified with the price
of capital services.36 An internally consistent treatment of the theory of
investment along the lines suggested by Klein leads to a comparative
statics theory of demand for capital services in which no demand func-
tion for investment goods exists.
Another branch of the current literature is based on the view that no
demand function for investment goods exists. We have already cited
Haavelmo's support of this position. A similar view may be found in
Lerner's Economics of Control. Lerner argues that, under diminishing
returns, the firm has a downward sloping demand curve for capital
services but that, except where there is no net investment, the rate of
investment is
• . there is no limit to the rate per unit of time at which [the individual]
.

can acquire assets by buying them, borrowing money for the purpose if he
has not enough of his own. This indefinitely great rate of "investment" means
that he can move at once to the position • which makes the (private)
. •

marginal productivity of capital equal to the rate of interest. Once he gets
there, there is no tendency for further expansion.
This view is the same as that expressed by Haavelmo. A recent restate-
ment of this position has been given by Witte, who concludes, with
Lerner and Haavelmo, that ". the continuous function relating the
. .

rate of investment to the rate of interest at the micro level has no
foundation in the ordinary theory of the firm."88 We have demon-
Klein, Keynesian Revolution, esp. pp. 196—199.
Klein, in Kykios, Vol. 2, fasc. 2, 1948, pp. 97—117; and his Economic
Fluctuations.
36 Ibid. The price of investment goods (p. 21 and p. 85) is identified with the
price of capital services (p. 15).
37 A. P. Lerner, The Economics of Control, esp. pp. 330—338.
38 James G. Witte, Jr., "The Microfoundations of the Social Investment Func-
tion," Journal of Political Economy, October 1963, pp. 441—456.

strated that it is possible to derive the demand for investment goods
from the comparative dynamics applied to the ordinary neoclassical
theory of the firm. The conclusion reached by Haavelmo, Lerner, and
Witte concerning a demand function for investment goods derived on
the basis of comparative statics is, of course, correct.
An attempt has been made by proponents of the view that the demand
function for investment goods does not exist to rehabilitate the Keynesian
marginal efficiency of investment schedule. Alternative versions of this
rehabilitation are presented by Haavelmo, Lerner, and Witte.39 The
essentials of the argument are that, at a given rate of interest, a certain
price for investment goods is required to equate the marginal produc-
tivity of capital with the implicit price of capital services; but the higher
this price the lower the rate of interest, so that a rising supply curve for
investment goods implies that the amount of investment goods produced
will increase as the rate of interest falls. A fundamental difficulty with
this view is that it fails to account for the purchase of new investment
goods by the users of capital equipment.4° Witte summarizes this conse-
quence of the view as follows: ". the rate-of-investment decision is
.

the rate-of-output decision of supplying enterprises and not the rate-of-
input decision of capital-using firms."4' In the same vein Haavelmo
writes, ". . . it is, actually, not the users of capital who demand invest-
ment, it is the producers of capital goods who determine how much
they want to produce at the current price of capital."42 A further attempt
along these lines of the rehabilitation of the Keynesian marginal effi-
ciency of investment schedule has been presented by Clower.43 His argu-
ment follows that of Haavelmo, Lerner, and Witte in assuming that
demand for capital services is equal to supply. However, Clower intro-
Haavelmo, Theory of Investment, pp. 194—197. See also: B. Thalberg, "An
Analysis of a Market for Investment Goods," in Lutz and Hague, Theory of
Capital, pp. 161—176, and "A Keynesian Model Extended by Explicit Demand and
Supply Functions for Investment Goods," in Stockholm Economic Studies, Pamph-
let Series, No. 3, 1964. Lerner, Economics of Control, pp. 333—334. Witte, in Journal
of Political Economy, October 1963, pp. 445—447.
40 A second difficulty with this view is that an increase in the price of invest-
ment goods may result in a rise or a fall in the supply of investment goods,
depending on the relative capital intensity of the investment goods and consump-
tion goods industries. Lerner, for example, assumes implicitly that investment
goods are produced with no capital services. This difficulty was pointed out to me
by James Tobin.
41 Ibid., p. 448.
42 Haavelmo, Theory of Investment, p. 196.
R. W. Clower, "An Investigation into the Dynamics of Investment," Ameri-
can Economic Review, March 1954, pp. 64—81.

duces a demand for investment goods which is not necessarily equal to
the supply of investment goods. The excess or deficiency of demand
over supply is net accumulation of capital. This view also fails to account
for the purchases of new investment goods by the users of capital
equipment.
For internal consistency, the rehabilitation of the Keynesian marginal
efficiency of investment schedule requires either a changing rate of
interest, as suggested by Haavelmo, or a changing price of capital goods,
as suggested by Lerner.44 For if the rate of interest and the price of
investment goods are fixed over time and the marginal productivity
of capital is equal to the implicit price of capital services, the firm's
demand for investment is determinate; this demand is precisely equal to
replacement demand so that net investment is zero. Under these circum-
stances, the rate of investment demand by users of capital equipment is
independent of the rate of interest so that the price of investment goods
must be that at which this rate of investment will be supplied by invest-
ment goods producers. But then if the marginal productivity of capital
is to be equal to the implicit price for capital services, the rate of inter-
est is uniquely determined, which is inconsistent with variations in the
rate of interest from whatever source.
To complete the rehabilitation of the Keynesian marginal efficiency
of investment schedule, interpreted as the level of investment resulting
from a market equilibrium in investment goods corresponding to a given
rate of interest, market equilibrium must be studied in a fully dynamic
setting. The demand for investment goods must be derived from a com-
parison among alternative paths of optimal capital accumulation. It
remains to be seen whether such a rehabilitation can be carried out in
an internally consistent way.
"Haavelmo, Theory of investment, p. 196. Lerner, Economics of Control, dia-
gram, p. 336.

COMMENT
ON CROCKETT-FRIEND AND JORGENSON
BY JAMES TOBIN, YALE UNIVERSITY

I agree with Jorgenson's general defense of the neoclassical theory of
the firm. As he says, its usefulness is by no means confined to static
conditions. As long as expectations are assumed certain, maximization
of the present value of the firm is as powerful a principle for dynamic
theory as profit maximization has been for static theory. A dynamic
theory based on this principle has much more to say, and can handle
many more complexities, than is often appreciated.
Jorgenson's specific example, however, is only barely dynamic. His
firm can maximize present value simply by maximizing profits at every
point in time. The firm confronts no intertemporal trade-offs, in which
profits now must be weighed against profits later. It purchases capital
services at a market rental, just as it purchases labor at a market wage.
There is a perfect market in capital goods; capital is homogeneous in
quality regardless of its vintage; and capital evaporates exponentially,
so that future depreciation is also independent of vintage. Thus, any
surviving capital can always be sold at the prevailing price of new capi-
tal goods. Therefore if, as Jorgenson assumes, the rental of capital
services correctly reflects interest, depreciation, and the change in price
of capital goods, the firm will be indifferent in choosing between renting
and owning. The present value of such future rentals just equals the
current price of capital goods.
I would like to make a parenthetical semantic remark: Jorgenson
calls the rental just discussed, specifically q(r + 6 — user cost.
To anyone who learned about user cost from the appendix to Chapter 6
of Keynes' General Theory, this terminology seems surprising. Keynes
assumed that the decline in the value of a stock of goods during a period
depends on the intensity of use, not just on the passage of time, hence
the term user cost. Keynes' assumption is notably absent from most
modern capital theory, including Jorgenson's. I find it confusing to see
a rental which is just a time or ownership cost called user cost.
By assuming diminishing returns to scale, Jorgenson makes the size
of his firm determinate within the framework of pure competition and
certain expectations. However, the sale of the services of owned capital
is an activity with constant returns to scale, and in Jorgenson's world of
perfect competition and perfect knowledge, the scale of ownership by
any one individual is indeterminate.

Comment 157
Given the time path of the price of the product p. the wage rate w,
and the rental on capital c, Jorgenson's firm decides upon the paths of
output Q, employment L, and use of capital services K. Indeed, these
paths wifi simply maximize profits pQ — wL — cK at each point in time,
subject to the production function. If the time paths of p, w, and c are
continuous, then so are the paths of Q, L, and K.
However, as Jorgenson points out, there is no reason to assume that
markets will never present an individual firm with jumps in p, w, and c.
If they do so, the firm's profit-maximizing response involves jumps in
Q, L, and K. In Jorgenson's firm there are no frictions or speed-of-
adjustment costs to make profitable any delay at all in responding to
new conditions.
Many economists—Jorgenson cites Haavelmo and Lerner—have
concluded that such an individual firm has no demand schedule for net
investment K but only a demand schedule for capital K. These theorists
think that if conditions change the optimal rate of use of capital services,
the firm will immediately shift to the new optimum—by renting more
or less capital or by buying or selling capital goods. This is not a sur-
prising conclusion. It is the use of capital services, proportional to the
stock not to the flow, which is related to the determining prices. Simi-
larly, the firm has a demand schedule for labor services, not for their
rate of change. No one is dismayed that a frictionless firm is expected
to shift in no time from one employment level to another.
The investment demand schedule which these economists have sought
and not found is a relationship at a given point in time t0 between
investment K(t0) (or K(t0) + at that Ume and the rate of
interest r(t0), holding constant all other current and expected prices.
This is the marginal efficiency schedule which Keynes purported to
draw at the aggregative level, which Lerner and Haavelmo doubted
existed for an individual firm, which Lerner tried to justify on macro-
economic grounds. Now varying r(to) to the individual firm, holding all
other relevant prices constant, is bound to cause the break-even rental
c(to) to vary also. Indeed all hypothetical values of r(to) except one
involve a jump at t0 in the optimal K(t). Moreover, one cannot escape
the conclusion that, except for the one value of r(to) which keeps c(t0)
adjusted to the existing capital stock, K(t0) must be either +co or —oo.
Jorgenson does not escape this conclusion either, but by asking a
different question he arrives at what he identffies as an investment
demand schedule. He does not hold all other prices, present and future,
constant while he varies r(t0). Instead he compensates the variation of
r(to) by changes in present and future q(t) so that c(to) remains the

Thus K(t0) is independent of these compensated variations in r,
but the subsequent path K(t) is not. And in particular K(t0) will
depend on r!
Maybe there is some question to which this is the answer, but it is
not the question to which Jorgenson finds previous answers so unsatis-
factory. There is no reason to assume that expected prices of capital
goods accommodate themselves so obligingly to interest rate variations.
Unless they do so, Jorgenson's investment demand schedule cannot
serve the analytical purposes for which such a schedule is desired, and
one must look elsewhere for a determinate theory of investment. At the
level of a single firm, this may be derived from frictional or adjustment
costs; at the level of the whole economy, it may be derived from capacity
limitations on production of investment goods (although here Lerner's
famous solution is, as Jorgenson points out, far from foolproof).
It would be desirable to have a neoclassical theory of consumer
investment to place alongside the theory of business investment. In such
a theory it would be necessary to state payoffs in utility rather than in
money, to recognize imperfections in rental and second-hand markets,
and to allow for true Keynesian user cost. A model of this kind would,
I think, suggest some differences between real and financial investments
by households which do not appear in the Crockett-Friend paper.
Their model is considerably less theoretical. In their view, each house-
hold has a desired total and composition of net worth, depending on its
normal income and its tastes, and on the yields and risks of various
assets and debts. Crockett and Friend explain flows of household invest-
ment and saving as a process of stock adjustment, without worrying
with Jorgenson why adjustments should not be instantaneous.
While I am sympathetic to this approach to empirical 'data, I think
the authors' formulation is too static. Not all flows should be interpreted
simply as efforts to eliminate discrepancies between actual and desired
stocks. Desired stocks change, and there would be nonzero flows even
if the household were continuously in adjustment. Even for the same
normal income, for example, a household's desired wealth will change
in total and in composition with time and age. I suspect that reformula-
tion along these lines might improve the authors' empirical estimates of
adjustment speeds, which are so far rather unsatisfactory.
The main purposes of the Crockett-Friend project, of which this con-
1 For example, if is increased for all
t to by the same amount as
a once-and-for-all rise in r at to,then c(to) is unchanged. Future c(t) are
increased, but since they are discounted more heavily their present value is still
q (t0).

Comment 159
ference paper is a progress report, are to estimate normal income elas-
ticities of demand for wealth and its components, and to estimate speeds
of stock adjustment. The data are cross sections, and the authors rely
especially on cross sections containing observations in the same house-
holds for more than one year.
A principal finding is that the ratio of net worth to normal income
increases with wealth and income. Crockett and Friend suggest that this
finding is inconsistent with saving theories which contend that "perma-
nent" saving is a constant fraction of permanent income. However, their
finding is relevant to this suggestion only when age is controlled. When
net worth and normal income are compared across age groups, wealth
will appear to have an income elasticity above one, even if saving does
not. Crockett and Friend do try to control for age, but their age brackets
are so broad as to leave the issue in doubt.
A potential test of great interest concerns households just retired or
about to retire. If those which had enjoyed larger earned incomes had
by this age accumulated relatively larger net worth, simple permanent
income models that assume all saving is for retirement would be called
into question. The Crockett-Friend findings for retired households do
appear to be inconsistent with those models and to suggest an estate
motive for saving. But these findings must be interpreted with caution
because of the vagaries of "income" reporting for persons already retired.
Other problems in interpreting the apparent high income elasticity of
demand for wealth arise from the possibilities that the net worth of high-
income households may be disproportionately swollen by inheritances
and unrealized capital gains.
In line with much recent work, Crockett and Friend devote consider-
able attention to the measurement of normal income free from transient
components. They use two devices—averaging of several annual incomes
reported for the household and averaging of incomes of members of an
occupational group. Neither device adds appreciably to the explanatory
power of two-year disposable income. However, calculations based on
groupings which allow for age are yet to be reported. When this is done,
it may be possible to use the age proffle of income for people with a
given occupation and education in computing their permanent incomes.
In principle, normal income should be forward looking not backward
looking.
So far the authors' calculations of speeds of adjustment are not very
encouraging. It is scarcely surprising that total wealth at the end of 1961
is related to wealth two years earlier. It is disconcerting that wealth at
the end of 1959 is not much help in explaining 1960-61 flows. With

respect to individual assets, few stocks were available for use in the
flow regressions, and these were used only in their "own" regressions.
Crockett and Friend are properly concerned with eliminating spurious
relationships due to persistent differences among households in "tastes"
—both general thriftiness and preferences for the services of particular
consumer assets. As is well known, these differences can obscure stock-
flow relationships in cross The authors' device of classifying
households by saving behavior in a year not used in the regression does
not, on the whole, produce significant results. It would be better, as far
as possible, to exploit the panel nature of the data to examine changes
in the behavior of identical households.
The paper is a progress report on a large-scale empirical research
project, and the main thing a discussant can do is to cheer the authors
on their promising line of inquiry. Perhaps it is not too irreverent for
this discussant, who has in the past labored in the same field, to remind
the authors of the challenge to all of us presented by the near con-
stancy of the aggregate ratio of household saving to disposable income
in the U.S. since the Korean War. Have our detailed researches yet
provided the forecaster and policy-maker with a better guide than the
rule of thumb that 5 per cent of disposable income is saved? Will they
give advance warning if and when this rule of thumb is breached? We
should gear our research to these questions and not be satisfied with
statistical explanations of household differences for their own sake.

ON CROCKETT-FRIEND AND JORGENSON
BY ZVI GRILICHES, UNIVERSITY OF CHICAGO
In the conventional approach, "theory" gives one the demand for
capital as a level and comparative statistics tells how much capital stock
will be demanded at different relative prices, but from neither can one
derive a unique optimal adjustment path from one equilibrium position
to another. There are two aspects to this position:
First, defining equilibrium as the stationary solution (dk/dt 0)
concedes the possibility that markets are out of equilibrium during the
investment process. Given full adjustment to the previous situation,
there is no positive net investment unless something (e.g., prices)
changes and disrupts the previous equilibrium. In this sense, net invest-
ment is viewed as a disequilibrium phenomenon.
Second, without adding additional constraints on the possible range of
adjustment or a concept of "cost of change," the instantaneous rate of

Comment 161
investment could be infinite in response to a once-and-for-all shift in the
exogenous variables.
Jorgenson's contribution, and it is an important one, is to show that
under certain conditions, when prices are and have been changing
smoothly, both problems need not arise and it is possible to derive a
unique relationship between the rate of investment and the variables
influencing it.
If things are continuously and smoothly changing, one may assume
that the firm is always in equilibrium—that all marginal conditions are
satisfied everywhere along the accumulation path. This allows one to
define different paths of accumulation and associate comparative
dynamic statements saying, e.g., that accumulations paths differing only
in the level of the ruling rate of interest can be characterized by larger
or smaller investment rates.
It should be pointed out, though, that the solution to these problems
is achieved through a very severe restriction on the scope of the original
question. In the Jorgenson model, one cannot answer the question of
what happens to the rate of investment if the rate of interest or other
prices shift to a new permanent level in one move or if a change occurs
in depreciation rules. A discontinuous jump to a new accumulation path
is not admissible. Since these are the types of questions that Haavelmo
and others wanted to answer, solving a more restricted problem, while
very useful, does not necessarily imply that they were wrong or that
their problem has been solved.
The conventional position, having got as far as theory would carry it
—to the demand for capital but not for investment, proceeded to "solve"
the problem by introducing ad-hoc "partial adjustment," "cost of
change," or "liquidity constraint" theories, which explained why and
how a particular desired change in capital levels is spread out over a
substantial period of time. The theoretical underpinnings of these addi-
tions were very weak, but they did force one immediately into a con-
sideration of lags and of a larger list of possible variables, making the
theory empirically much more promising and effective.
By limiting himself only to continuous changes, Jorgenson shows
that this type of ad-hockery is redundant as far as the original problem
is concerned. It can be solved in a smooth world within the original
theoretical model without invoking various dubious lag hypotheses. But
this may be an illusory gain. To be effective econometrically, the Jor-
genson theory will also have to be broadened to include some lag or
"cost of change" hypotheses. As of now, it implies that dk/dt (net
investment) = 0, whenever wages, prices, or interest rates are constant,

irrespective of their previous paths. Adding some lag hypothesis did
solve the infinite derivative problem in the original model. Since some
lag hypothesis will also be necessary in this model, it is not all that clear
what will be the final contribution of solving the infinite derivative prob-
lem separately. It is clear, though, that the comparative dynamics
apparatus developed by Jorgenson will prove very useful in future
elaborations of this and similar models. A very important problem still
remains unsolved, however: the form and determinants of the optimal
adjustment path from one equilibrium position to another. We hope to
be able one day to derive it as an implication of our theoretical model,
instead of just tacking on something "reasonable" at the end. Showing
that these lag hypotheses are not necessary to solve one problem (the
derivation 'of an internally consistent investment function) does not
make them any less important.
I have only a brief comment on the Crockett-Friend paper. Their
theory should allow for a replacement component of saving, since their
saving is gross saving (at least in some of its components). Thus, the
coefficient of assets is equal to the difference between the rate of depre-
ciation (replacement) and the rate of adjustment. This may explain why
they get, on the face of it, such unreasonably low estimated rates of
adjustment. One should add to these the appropriate average mainte-
nance and replacement coefficient associated with the given level of
assets.

ON JORGENSON
BY ROGER F. MILLER, UNIVERSITY OF WISCONSIN
Jorgenson's paper does a great deal to expose the misunderstandings
at the heart of the controversy on whether or not an investment demand
function is derivable from the neoclassical theory of the firm. In brief,
the neoclassical theory contains a demand-for-capital-services function;
to get capital services, the firm acquires capital assets (or another firm
acquires them and rents them to the producing firm); and acquisition
of additional capital assets is defined to be gross investment. The demand
for investment is derived from the demand for capital assets, which in
turn is derived from the demand for capital services. There are, thus,
three demand functions involved, all intimately related, and either all
exist or none exists. The existence of any one is unaffected by the fact
that it may be a simple transformation of another in a simple model.
Nor is it affected by the fact that it is a derived demand. Most demands
are "derived"! It may be that there is little point to introducing the

Comment 163
concept of investment in such a model, but this is a distinct objection
unrelated to the controversy.
More consequential is the problem of the continuity and continuous
differentiability of the demand-for-capital-assets function. At any point
where this function is not continuously differentiable, the investment
function becomes discontinuous. If the demand-for-capital-assets func-
tion is also discontinuous, the fact that the neoclassical model allows
instantaneous adjustments has been interpreted as implying that the
amount of investment at such a point is unbounded when expressed as
a rate per instant of time. Jorgenson's paper adds nothing to the solu-
tion of this problem because he merely finesses the problem completely.
Following the apparent intent of the neoclassicists, Jorgenson makes
adjustments instantaneous, and he also imposes continuity on the vari-
ables he discusses. In particular, his introduction of K(t) in (3) and its
treatment in the present value maximizing exercise which follows is
tantamount to assuming that K(t) is continuous and differentiable from
the beginning. His later interpretation of condition (3) is less tha
helpful because it seems to imply that the assumed continuity is a
result of the analysis. In condition (3) Jorgenson defines investment as
"1(t) = K(t) + 6K(t), where K(t) is the time rate of change of the
flow of capital services at time t." If K(t) is not differentiable or is dis-
continuous at t, this is inappropriate because K(t) is undefined. Jorgen-
son has simply assumed that such occasions do not arise, and thus sheds
no light on this aspect of the controversy.
Jorgenson's contribution is interesting and valuable in spite of his
having finessed the unboundedness issue, to which I will return below.
It is, however, unfortunate that Jorgenson muddied the waters by dis-
cussing, however briefly, the arguments and evidence for aggregate
investment functions, which might be very closely approximated by
continuous functions even if firm plant investment functions are not,
but which are at best very tenuously related to the microfunctions men-
tioned in the first paragraph above. Apart from this, I believe the
Jorgenson paper is a worthwhile opposite extreme to the case of once-
and-for-all adjustment where the capital stock for a given "firm" is a
fixed amount.1 In the latter case, it is clear that in determining the
initial (and permanent) capital stock of a given plant, the amount of
capital (and thus the amount of investment in that enterprise) is nega-
'See Vernon L. Smith, "The Theory of Investment and Production," Quarterly
Journal o/ Economics, February 1959, and Roger F. Miller, "A Note on the
Theory of Investment and Production," Quarterly Journal of Economics, Novem-
ber 1959.

tively related to the rate of interest and to the cost of capital assets.2
Because this conclusion carries through in Jorgenson's instantaneous
adjustment model, there is a strong presumption in favor of this relation
holding for intermediate lagged adjustment cases.
While I welcome Jorgenson's explication of the neoclassical frame-
work, I feel that one of his contributions is exposing the rather severe
restrictions one must impose upon the model in order to deal with some
of the important questions of concern to economists. I strongly doubt
that the prominent neoclassicists, were they alive and well read today,
would find much interest in a model which assumes away uncertainty
with regard to the future, lags in adjustment, difficulties of aggregation
and composition, discontinuities, etc. Jorgenson's analysis should help to
bury this Caesar, as well as praise him.
I believe the day has passed when our analyses have surpassed our
observational and computational capabilities. Jorgenson's introductory
remarks are to the point. It is just because of this that I think it is
unfortunate that Jorgenson chose to sidestep the unboundedness prob-
lem. I wish to make it clear that I am not concerned with the "realism"
of the model, but with the domain of its application. For most purposes,
it may be perfectly satisfactory to regard a fully continuous model as a
sufficient approximation to our essentially discrete activities. I strongly
suspect that not all purposes are served equally by this approximation,
and that for investment timing for a particular firm or individual dis-
continuities may be of the essence. If this is so, the relevant discontinui-
ties should be recognized and the model constructed so as to allow for
them.3 In the conference discussion it was pointed out that adoption of
"period analysis" using discrete time intervals, or of a lagged adjustment
function, represent two ways of avoiding the discontinuity problem.
Neither of them is fully consistent with the instantaneous-adjustment
full-equilibrium framework of the neoclassicists, however, and both
merely sidestep the controversy in another dimension. Because both
Jorgenson and the discussants thus leave the controversy in an unsatis-
factory state, I should like to put forward a few comments and sugges-
tions on dealing with nondifferentiability and discontinuity problems
which I hope may resolve the present controversy and have much wider
applicability as well.
As a preface to my suggestions, I feel it is necessary to point out
that the concept of a function is independent of the concepts of differen-

2 The result comes from finding and from (68) in Miller
(ibid., p. 678).
3 This is, of course, one of the principal motivations of the model presented in
the Miller-Watts paper included in this volume.

Comment 165

liability, continuity, or boundedness, although the latter properties make
functions more tractable to traditional mathematical manipulations.
Thus, to say that the investment function is unbounded at any time for
which the demand-for-capital-assets function is discontinuous does not
imply that the investment function does not exist. However, it does
raise questions as to the economic sense of the function as it is defined.
We do a disservice to the science of economics (and to the discipline
of mathematics as well) if we bind ourselves too rigidly to conventional
and convenient mathematical formulations and definitions. I believe
this is precisely the heart of the problem in this controversy: it is much
ado about nothing, where the nothing in question is the time between
t and t (i.e., dt = 0). In particular, there is a confusion between the
instantaneous time rate of change of capital assets and the quantity of
investment which takes place at any instant. The latter is clearly what
we are interested in; the former is useful only if it leads to the latter.
Without loss of generality, the ensuing discussion is simplified and
clarified by assuming that we have the following function for the quan-
tity of capital assets demanded as a function of the continuous variable t
over the interval from t = 0 to t = 4:

(a) K(t) = 2.Ot for 0<t<1.
(b) K(t) = 2.0 — 0.5(t — 1) for I < t < 2. (I)
(c) K(t) = 4.0 + 0.2(t — 2) + 0.1(t — 2)2 for 2 t 4.
The units in which K(t) is measured are whatever is appropriate for the
way K is defined, say, tons of machinery. This yields the following
diagram:

K(t)
5

4

3

2

/
0 1 2 3 4

FIGURE 1

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